1. Field of the Invention
The present invention relates to a magnetic transfer master carrier for magnetically transferring information to a magnetic recording medium, a magnetic transfer method using the magnetic transfer master carrier, and a magnetic recording medium produced through the magnetic transfer method.
2. Description of the Related Art
As magnetic recording media capable of recording information with high density, magnetic recording media of perpendicular magnetic recording mode (hereinafter referred to as perpendicular magnetic recording medium or media) are known. An information recording area of a perpendicular magnetic recording medium is composed of narrow tracks. Thus, as to the perpendicular magnetic recording medium, a tracking servo technology plays an important role in performing accurate scanning with a magnetic head within a narrow track width and reproducing a signal at a high S/N ratio. To perform this tracking servo, it is necessary to record servo information, for example a servo signal for tracking, an address information signal, a reproduction clock signal, etc. as a so-called preformat, at predetermined intervals on the perpendicular magnetic recording medium.
As a method for preformatting servo information on a perpendicular magnetic recording medium, there is, for example, a method wherein a master carrier having a pattern consisting of a plurality of convex portions having a magnetic layer on their surfaces, which corresponds to the servo information, is closely attached to the perpendicular magnetic recording medium, and then a recording magnetic field is applied there-to so as to magnetically transfer the servo information corresponding to the pattern of the master carrier to the perpendicular magnetic recording medium (see, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-203325 and 2000-195048 and U.S. Pat. No. 7,218,465, for example).
In this method, when a recording magnetic field is applied to the perpendicular magnetic recording medium, with the magnetic master carrier and the perpendicular magnetic recording medium closely attached to each other, a magnetic flux is absorbed into the patterned magnetic layer based upon the magnetized state of the master carrier. As a result, the recording magnetic field is increased in strength according to the pattern of the master carrier. The magnetic field increased in strength in the form of the pattern enables to magnetize only predetermined regions of the perpendicular magnetic recording medium. Thus, the servo information corresponding to the pattern of the master carrier is magnetically transferred to the perpendicular magnetic recording medium.
After the magnetic transfer, the recording magnetic field is cancelled, and the master carrier which has been closely attached to the perpendicular magnetic recording medium is separated therefrom.
Conventionally, magnetic materials with high saturation magnetization have been used as materials for magnetic layers of master carriers of this type. This is because when a recording magnetic field is applied, the magnetization of the magnetic layer of the master carrier is increased so as to easily absorb a magnetic flux in the magnetic layer.
However, the magnetic layer of the master carrier is very thin, specifically, roughly several tens of nanometers in thickness, thereby strongly influenced by a demagnetizing field. Therefore, even if a magnetic material with high saturation magnetization is used as the material of the magnetic layer, an effective magnetic field (recording magnetic field) applied to the magnetic layer decreases due to the demagnetizing field, and the magnetic layer becomes unsaturated. As a result, inconveniently, the magnetization of the magnetic layer cannot be increased as much as desired.
Thus, as a material for the magnetic layer, use of magnetic materials with perpendicular magnetic anisotropy, which is hardly influenced by a demagnetizing field, is studied.
Although the magnetic layer composed of a magnetic material with perpendicular magnetic anisotropy is hardly influenced by a demagnetizing field, it causes a problem that a coercive force He and a residual magnetization Mr are large.
When the coercive force Hc is large, it is necessary to increase the strength of an applied magnetic field required for saturating the magnetic layer. However, due to the increased strength of the applied magnetic field, the perpendicular magnetic recording medium tends to be magnetized by a magnetic field present in regions other than the magnetic layer in the master carrier.
When the residual magnetization Mr is large, even a small shift of the position of the master carrier toward the surface of the perpendicular magnetic recording medium is likely to cause unnecessary magnetization of the perpendicular magnetic recording medium due to a residual magnetization of the magnetic layer in the master carrier, at the time of the separation of the master carrier from the perpendicular magnetic recording medium.